Retrofocus-type wide-angle camera lens

ABSTRACT

A RETROFOCUS-TYPE WIDE-ANGLE CAMERA LENS IS PROVIDED IN WHICH THE BACK FOCUS IS ESPECIALLY LONG COMPARED TO THE COMPOSITE FOCAL LENGTH. THE LENS COMPRISES A FRONT LENS GROUP HAVING A NEGATIVE COMPOSITE FOCAL LENGTH AND A REAR LENS GROUP CONSISTING OF A CEMENTED POSITIED LENS, A NEGATIVE SINGLE LENS, A COMPOSITE POSITIVE MENISCUS LENS, AND A SINGLE POSITIVE LENS. TGHE LENS THUS PROVIDED HAS AN APERTURE RATIO OF F/2.8 AND A VIEWING ANGLE OF OVER 80* WITH VARIOUS ABERTIONS, COMA, CURVATURE OF THE FIELD AND DISTORTED HIGHLY CORRECTED.

1973 YOSHIYUKI smmrzu 27,799

RETROFOCUS-T'YPE WIDE'ANGLE CAMERA LENS Original Filed Dec. 24, 1969STOP L7 L8 L9 5 F6 Ha Ha Us We w. k "QOZITHTI 0.02 0.02mm 0.02 "2%SPHER'CAL ABERRAT'ON ASTIGMATISM DISTORTION SINUSOIDAL CONDITION UnitedStates Patent 27,799 RETROFOCUS-TYPE WIDE-ANGLE CAMERA LENS YoshiyukiShimizu, Tokyo, Japan, assignor to Nippon Kogaku K.K., Tokyo, JapanOriginal No. 3,622,227, dated Nov. 23, 1971, Ser. No. 888,041, Dec. 24,1969, which is a continuation-in-part of Ser. No. 611,206, Dec. 27,1966, now abandoned. Application for reissue Sept. 13, 1972, Ser. No.288,502

Claims priority, application Japan, Dec. 28, 1965, 41/ 80,685 Int. Cl.G02b 9/64 US. Cl. 3502I4 2 Claims Matter enclosed in heavy brackets [1appears in the original patent but forms no part of this reissuespecification; matter printed in italics indicates the additions made byreissue.

ABSTRACT OF THE DISCLOSURE A retrofocus-type wide-angle camera lens isprovided in which the back focus is especially long compared to thecomposite focal length. The lens comprises a front lens group having anegative composite focal length and a rear lens group consisting of acemented positive lens, a negative single lens, a composite positivemeniscus lens, and a single positive lens. The lens thus provided has anaperture ratio of F/2.8 and a viewing angle of over 80 with variousaberrations, coma, curvature of the field and distortion highlycorrected.

This application is a continuation-in-part of my copending applicationbearing Ser. No. 611,206 filed on Dec. 27, 1966, now abandoned.

This invention relates to a retrofocus-type wide angle camera lens.

In a single lens reflex camera, the back focus of the objective lens isrestricted because of the operation of the reflecting mirror, and it isnecessary that the back focus (Bf) of the objective lens of, forexample, a 35 mm. camera should be longer than about 35 mm. Under suchrestrictions, it has been difficult to obtain a super wide angle lensbecause distortion and coma aberration are increased as the angle ofview is increased although a retrofocus type objective lens hashithereto been used in order to obtain a wide angle objective lens whosefocal length was shorter.

On the one hand, the increase in spherical aberration is not seriousinsofar as a wide angle lens is concerned, but the increase ofastigmatism and distortion is detrimental to the quality of the image,and therefore the correction of these aberrations must be solved.

The object of the present invention is to provide a retrofocus-type wideangle lens in which astigmatism and distortion are highly corrected.

In accordance with the present invention, there is provided aretrofocus-type wide angle camera lens in which the back focus isespecially long compared to the composite focal length, an apertureratio of F/ 2.8 and a viewing angle of over 80' being provided and inwhich the various kinds of aberrations, in particular, coma, curvatureof field and distortion are highly corrected.

Other objects, advantages and features of the present invention willbecome more apaprent from the following "ice description of anillustrative embodiment in conjunction with the drawing in which FIG. 1is a cross-sectional view of retrofocus-type wide angle lens accordingto the present invention; and

FIG. 2 shows the various aberration curves of the lens illustrated inFIG. 1.

As viewed from the direction in which light rays are incident, L is anegative single meniscus lens with the convex surface directed towardsthe object, L is a positive single lens, and L is a negative meniscuslens with the convex surface directed towards the object, the threelenses comprising the front group of the lens, the composite focallength of which is negative. Separated by a comparatively large airspace d from the lens L is a cemented positive element comprising apositive lens L and a negative meniscus lens L Separated by air space dis a negative lens L followed by a composite meniscus lens formed by anegative lens L and a positive lens L A single positive lens L completesthe rear group.

The lens arrangement is such that the following conditions are met:

Where d and its subscript designate distance along the optical axis andn and its subscript designate the refractive indices designated on thedrawing and hereinafter fully set forth. The present invention ischaracterized by the above mentioned conditions, and the gist of thepres ent invention, resides in the fact that the compositic focal lengthof the front group lenses L to L is made negative to form a virtualimage of the object, the virtual image being formed at an indefinitedistance by means of the group of lenses L to L (Tessa-type lens isprovided in the opposite direction, and single convex lenses are madeinto a cemented lens of L and L By means of lens L the real image isformed at a predetermined position. In order to obtain an image whoseaberrations are highly corrected it is necessary to correct variousaberrations of the first virtual image, and for that purpose, thecorrection of chromatic aberration is carried out by constituting lensL, a negative lens. Other aberrations such as spherical aberration,coma, image curving, distortion, and the like, are not highly corrected,but these aberrations are sufficiently corrected by the rear group oflenses.

The lenses L L L L L and L are lens groups having a positive refractivepower and the groups of lenses L to L has a shape and refractive powernearly equal to that of the known Tessa, or triplet lens type arrangedoppositely. This aims to focus at infinity the virtual image formed bylenses L to L through the aid of lenses L to L The lens L gives apredetermined focal length to constitute the whole lens system.

A Tessa-type lens is principally a symmetrical lens type although itssymmetry is somewhat deformed. The central negative lens is provided forcorrecting spherical aberration and axial chromatic aberration and astigmatism. For this purpose, it is most effective to place it at thecentral position, since the negative lens if located at the centralposition, has little effect on distortion and magnification chromaticaberration. Lenses L to L adopt this idea of the Tessa-type and this isexpressed as 0.5dg d11 2d9. With this condition, the negative lens Lshould be placed substantially intermediate of the positive lens groupsL L and L L In other words, the condition(l) is effective for producinga plane image, and it is the condition for placing the lens L almost inthe middle between the composite lens L L an composite lens L L When soarranged, the main light rays pass through almost the center of lens L(near to the optical axis), minimizing the adverse effects ofastigmatism, at the same time constraining spherical aberrations. Whensuch conditions herein pro posed is changed, the main light rays passthrough the periphery of L, (the marginal portion of lens), and not onlyastigmatism but also the spherical aberration is increased.

The object of the condition (2) is to correct distortion and theinternal coma generated by the oblique light rays, i.e. the light rayscoming into the system from the outside of the main light rays of thelight coming in at a predetermined angle against the lens, suchaberration being experienced in the lens systems of this kind. In otherwords, surface r of lens L is formed in the negative curved surface, andtherefore the dilference of the angle of incidence caused by thedifference of the height of incidence of said oblique light rays,becomes larger. Thus the oblique light rays are greatly refracted andthe internal coma generated by the front group of lenses L to L can besufiiciently corrected. If the condition is reversed, the internal comagenerated in the front group of lenses is increased, and correction bythe rear group of lenses becomes difiicult.

Lenses L L and L constitute a lens group having a negative refractivepower. However, when the position of the entrance pupil is behind thelens group, the negative lens has a tendency to coma aberration. In thisinvention, the negative lens group of L L and L produces internal comaaberration, which must be compensated. Condition (2) was determined forcorrecting the internal coma produced relative to the light raysincident at a position far away from the optical axis. The stop of thelens system according to this invention is behind the lenses L and L sothat when the light rays pass through r the light rays receive largerefraction due to the negative radius of curvature of r and refractiveindexes n n so as to approach to the main light rays. Since the internalcoma aberration of the light rays incident externally of the main lightrays is caused by the refraction of said light rays in the directiondeviating too much from the main light rays, the above-mentionedcondition plays a role to compensate for such coma aberration.

The object of the condition (3) is to correct the internal comagenerated by the aslant light rays, i.e., the light rays coming into thesystem from inside of the main light rays coming in at a predeterminedangle against the lens. In other words, the angle of incidence of theaslant light rays onto surface r of lens L is larger than the angle ofincidence of the aslant light rays coming onto surface r of lens L andthe refraction by the surface r becomes larger than the refraction bythe surface r However, when the direction of the sign of inequality isreversed while keeping the sum of the refractive powers of r and rconstant, the change of Petzvals sum is small, and therefore theeffectiveness in correcting astigmatism is decreased. The difference ofrefraction caused by the difference of the height of incidence of thelight rays passing through lens L is further decreased and therefore itis useless for the correction of the aberration.

Condition (3) is then to compensate the internal coma aberration of thelight rays incident at the position nearer to the optical axis than themain light rays. In this invention, the stop is positioned intermediateof lenses L and L so that the light rays incident internally of the mainlight rays will pass lens L externally of the main light rays and theheight of said light rays passing through lens r becomes higher thanthat of the light rays passing through the lens r Since r has a positiveradius of curvature, the negative lens L should bend toward the imagefield side, so that the light rays receive a large refraction by r todepart from the main light rays. This results in the correction of theinternal coma aberration. This effect is further assured by thecondition (I), i.e., 0.5dg d11 2d9, which assures passage of the mainlight rays through substantially the central portion of the lens L Thelight path thereof is not effected too much by the bending of the lens LThe object of the condition (4) is to attain the same object and effectof the condition (3). Namely, surface r of lens L corrects the internalcoma of the light rays coming into the system from inside of the mainlight rays of the light flux coming in at a predetermined angle, but theinternal coma is further corrected by the surface r between lenses L LThis correction is carried out in such a manner that it increases therefractive index In, of the negative lens L and decreases the refractiveindex 11 of the lens L whereby the curving in the positive directiongenerated on the sagital image surface is compensated in the negativedirections. If the direction of the sign of inequality is reversed, theangle of incidence of the oblique light rays becomes smaller and becomesuseless for the correction of the internal coma generated in the frontgroups.

Condition (4) is thus determined for achieving two purposes. As is wellknown, it is necessary to make the refractive index n of the negativelens L larger than n of the nositive lens L so as to curve the sagitalimage plane toward the object side, i.e., negative. The other purpose isto compensate the internal coma abberation due to r of the light raysincident internally of the main light rays. The result of this conditionis substantially the same as for r,,, i.e., the cemented surface oflenses L and L The image of the object is formed at almost indefinitedistance through lenses L to L when the above given conditions aresatisfied, but it is not always necessary to form the image of theobject at the indefinite distance. When the absolute value of thecomposite focal length of the lens system from L to L is less than twicethe composite focal length of the whole lens system, and is in thepositive, the composite focal length of the lens system from L to Lbecomes shorter, and the correction required becomes excessive, and thecorrection of aberrations difiicult. On the other hand, when it is anegative value, it would be necessary for lens L to have remarkablygreat refractive power. This also brings about poor balance, and thecorrection of aberration becomes difllcult. Therefore, the compositefocal length of the lens group from L to L should be preferably morethan twice as much as the composite focal length of the whole lenssystem, and the virtual image or the real image formed at the indefinitedistance can be formed as the real image at the rear of the lens bymeans of the lens L As a matter of fact, in addition to the conditiongiven above, various kinds of conventional means for correctingaberrations are employed, and as a result, a retrofocus-type wide anglecamera lens, having an aperture ratio of F/2.8, an angle of view of over80, the back focus of which is over 1.5f, and which has been highlycorrected for coma, image-surface curving and other aberrations, hasbeen obtained.

The following are the examples of the present invention.

indices of the respective elements and vd the Abbe numbers for theglasses of respective elements.

Seidel aberration coeflicients of Example I are given below:

r I II III-IV IV V 1 -0.0151218 0.0243690 0.0785414 0.1024190 0.01008422.- -1.2110050 0.1040224 -0.0504430 -0.4550300 0.0095350 3.- 0.10401100.1327508 0.1810070 0.1020328 0.1320120 4.- 0.0020300 -0. 0.07000040.1375074 -0.5740002 5.. 2007854 0.1225240 0.1112800 0.0019420 0.10437700.. -14.7832940 1.0407002 -0.5127082 1.0917724 0.1437004 7.. 8.20408701.0188750 0.2502072 0.4880270 0.0000550 5.. 1.1427144 0.37032380.2400244 0.17344778 -0.0502104 0.. 2.4505012 -1. 1583944 1.00115220.7003018 -0.a075034 1 -0.8247514 0.5720000 -0.7042440 0.52120000.3017100 11. -1.9778536 0.0053438 1.001s022 0.8205012 -0.4150774 12.0.0175012 0.0588448 0.3057122 0.1000100 0.3005872 13- -1.0245810-0.5332500 0.0502104 0.2235402 0.0730800 14.. 4.5082024 -0.00474080.1001002 0.5305102 -0.0714104 15. -0.0005000 0.0000302 -0. 1488550-0.0211022 0.2300840 4.4003000 -0.718814B 0.2340308 0.3370502 0.0550502EXAMPLE I In accordance with the present invention described it is t=1.0111-15507 2n=84 F:2. possible to produce a small, light-weight,retrofous-type nd 170. wide angle camera lens wherein the angle of viewis over 80, and F is 2.8, and the back focus B.f is over 1.5 times =+g mno more than the composite focal length, and in which the n=+03958d|=m125 14152041 varlous kinds of aberrations are highly corrected overd,=0.2025 the whole angle of view. When the lens system of the 11::022921:11.204. 13 present invention is mounted in a single lens reflexr.=-a.7500 camera, it is not necessary to elevate the reflectingmirn=+lwm ror in advance. The lens system of the present inventiond.=0.1125 n;=l.62041 60.3 can be incorporated into a camera in the samemanner dFMm as a conventional lens system. r =-|l.0521 004 36 a What isclaimed is: n= o.m2 di='1667 1. A retrofocus-type wide angle camera lenssystem,

d,=0.1250 m=1.51823 59.0 of which the numerical data is as follows:=-1.4542

3 m7 dn= 0 Focal length f=1.0 angs =1 7347 Back focal length B.f=1.5597m=+10500 d was Angle of field 2=84 m= 5 0000 Aperture ratio F:2.8

d,,=0.0417 n 1.7047 20.1 =+1.1729 d =0.2250 n =l.6204l 00.3 I|4=0.8333 d00042 n= =1.0 B.tl.5507 2 :2. n.=+s.0000 f n 04 r d=0.1450 n.=1.744 44.0m vd r1 =1.9423

45 r =+2.5000 EXAMPLE 11 +0 8958 d,=o.1125 m=1.02041 00.3 l a 1 1.0 Br15032 20 so 0 (1602625 l.3=+3.7500 nd vd d|=0.2292 n,=1.02041 00.3

Wham) d 00042 1: +2.0873 r =+1.2500

d =0.l80 n =1.62041 60.3 =0 1125 n =1 62041 &3 n=-|-l).7842 r =+0.4583

+4 6000 dg=0.3UD +1 0521 d =0.40B3 r l'7= d;=(].552 I14=I1J3228 56.9ym57 =pggm4 3 ,3 =-2.0420 n=0.7742

d =0.004 1 42 a =0.1250 n =l.5l823 50.0 I

mz n,=1.58013 01.2 45 d,=0.1042 =4 0.4520 3.5417

0 8 00 d.=0.202 d =0.2208 11.: 1.7847 r 1.0500 d =0.200 n =1.57501 41.3du=0,091g r =(].84(]0 l'12="5.0D00

d|=0.054 m= 60 1.,=0.0417 m=1.7047 20.1 n=-l.4709 r, =+1.1720

d,=0.008 =0.2250 m=1.02041 00. :1 I1p=40.0000 h4= -0.8333

d =0.160 n =1.51B23 50.0 du=om4z 11=+ m=+8.0000

0 9200 d11= 23 d,,=0.1458 =1.744 44. 9 r 4 d,,=0.028 n =1.7847 20.1 19 r1.1700

d,,=0.100 n =1.6679 55.5 Iu=0.7440

d1 =llm4 n =8.8000

d =0.l04 n =1.766B4 40. 2 H m=0. wherein r r represent the radn ofcurvature of the respective elements; d d; represent the axial thick- Inthe above examples, r r represent the radii nesses of the respectiveelements and the air spaces of of curvature of the respective elements,d d, the the adjacent elements; 11 112 represent the refracaxialthicknesses of the respective elements and the air spaces of theadjacent elements, n 0, the refractive tive indices of the respectiveelements; and vd represent the Abbe Numbers of the respective elements.

2. A retrofocus-type wide angle camera lens, of which the numerical datais as follows:

Focal Length r=1.o

Back Focal Length B.f=l.5032

Angle of Field 2'=80 Aperture Ratio F:2.8

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

FOREIGN PATENTS 6/1964 France 350214 3/1965 France 350214 JOHN K.CORBIN, Primary Examiner

